The invention relates to a method for purifying phosphorus, as a result of which the area of application of products produced from phosphorus can be expanded due to a low sulfur content.
Generally, organic impurities up to 0.3% are contained in technical grade white phosphorus. These impurities interfere with further processing and must therefore be removed before subsequent reaction. According to GDR patent 76,482, removal of the interfering organic impurity is possible by a treatment of the phosphorus with sulfuric acid. The phosphorus to be purified is intensively mixed briefly with highly concentrated sulfuric acid in a special apparatus. During this mixing, an emulsion is formed. In order to separate the concentrated sulfuric acid rapidly from the emulsion, the latter is passed into warm water. A rapid separation of the emulsion is required in order to avoid explosive reactions between phosphorus and sulfuric acid. Due to its higher specific weight, the purified phosphorus sinks countercurrently to the warm water that is being passed in and, to remove occluded sulfuric acid, is passed through a packed column and is subsequently collected in a tank of about 20 m.sup.3 capacity that is disposed below the column. Here the phosphorus is stored, frequently for several days, before it is used.
The approximately 5 to 10% sulfuric acid solution, which furthermore results from the emulsion, is largely removed as effluent from the plant. Due to back-mixing which sets in, the concentration of the sulfuric acid in the column and in the tank increases very quickly. Here also, it reaches concentrations of about 5 to 10%.
The purifying effect achieved with this procedure is good. For some applications, however, the relatively high increase in the sulfur content of the phosphorus, which is purified in this manner, results form the reaction of the sulfuric acid with the phosphorus during the purification procedure and is a disadvantage. In general, concentrations of 500 ppm and, in rare cases, concentrations even as high as 2,000 ppm are achieved. In a side reaction, a small portion of the sulfuric acid is reduced by the phosphorus and therefore leads to an increase in the sulfur content of the phosphorus. In certain applications, the high sulfur content leads to undesirable side reactions. For example, hydrogen sulfide is formed as a by-product during the processing of the phosphorus to phosphane. It is also disturbing that such phosphorus, upon contact with water, gives off poisonous hydrogen sulfide and thereby makes handling and transport more difficult.